In a conventional automotive air conditioning system of the type which includes an automatic temperature control function, a temperature control circuit is normally provided to control the angular position of an air mix damper to thereby control the temperature of the air blown into a compartment. By controlling the angular position of the air mix damper, the temperature control circuit adjusts the mixture of cool air and heated air directed to the compartment. The temperature control circuit computes the desired open angular position of the air mix damper as a function of a predetermined temperature, an outside air temperature and an inside air temperature.
The air mix damper generally is rotatably driven by a drive device such as a motor actuator which is responsive to control signals generated by the temperature control circuit. However, in rotating the air mix damper to a predetermined angular position, the motor actuator is subject to upper and lower rotational limits corresponding to the permissible range of rotation of the air mix damper. The motor actuator includes a drive motor, a regulator which regulates the angular position of the air mix damper in accordance with these upper and lower rotational limits and a position detecting device, such as a variable resistor, which generates position signals provided to the temperature control circuit and corresponding to the rotational limits of the air mix damper.
In the above prior art motor actuators, a relative position lag occurs between the position signals provided by the position detector or variable resistor and the upper and lower rotational limits of the regulator. In addition, the resistive value of such variable resistors is not always certain. Accordingly, a well-known technique to improve the accuracy of the position signals is to forcedly operate the motor actuator upon turning on the air conditioning system. The position signals of the motor actuator at the upper and lower rotational limits of the regulator are input to the temperature control circuit, and used to control the angular position of the air mix damper.
Despite the above improvement, differences in the stroke of the air mix damper and the motor actuator still occurred since the lengths of the rods and links which connect the air mix damper with the motor actuator could not always be precisely determined. Furthermore, if an error occurred upon connection of the air mix damper to the motor actuator, the motor actuator might not be driven to its upper or lower rotational limits even if the air mix damper is completely closed or open. In this case, the drive motor of the motor actuator could lock, and thereafter the temperature control circuit could not control the motor actuator. Additionally, the drive motor could be destroyed if current is continuously supplied to the motor over a long period of time.